An ignition plug is attached to an internal combustion engine or the like and used for igniting an air-fuel mixture or the like in a combustion chamber. Generally, the ignition plug includes an insulator having an axial hole extending in the direction of an axial line; a center electrode inserted into a forward end portion of the axial hole; a metallic shell provided externally of the outer circumference of the insulator; and a ground electrode fixed to a forward end portion of the metallic shell. The insulator is inserted into the metallic shell along the inner circumference of the metallic shell; then, a rear end portion of the metallic shell is bent radially inward to form a crimped portion, whereby the insulator is fixed to the metallic shell. Additionally, a gap is formed between a distal end portion of the ground electrode and a forward end portion of the center electrode. A high voltage is applied to the gap for generating spark discharge, thereby igniting the air-fuel mixture or the like.
Incidentally, application of voltage to the gap may be accompanied by formation of an electric field having high intensity at the crimped portion. The formation of an electric field induces a local breakdown of gas existing around the crimped portion between the crimped portion and an ionization of the gas existing around the crimped portion, potentially resulting in such generation of corona discharge as to creep along an outer circumferential surface of the insulator from the rear end of the crimped portion. The generation of corona discharge does not raise any particular problem in ignition performance. However, for a device that detects the condition of combustion of an air-fuel mixture or the like and the condition of generation of knocking by detecting ionic current that flows across the gap as a result of combustion of the air-fuel mixture or the like, the generation of corona discharge generates noise in the ionic current, potentially resulting in deterioration in accuracy in detection of the condition of combustion, etc.
Thus, in order to restrain the generation of corona discharge, there are proposed a method in which a filling layer having a relatively large resistance is provided between the crimped portion and the insulator and a method in which an electrically conductive coating electrically connected to the metallic shell is provided on that region of the insulator which faces the crimped portion. For example, refer to Japanese Patent Application Laid-Open (kokai) No. H11-233234 (Patent Document 1).